One other thing, but it's expensive.
Colt now offers the LE901 in .308 (7.62X51)
This lower will accept a Colt 5.56X45 AR-15 upper, so you can have both uppers on one lower.
It's a solution to the problem, but it is mighty expensive, and AFAIK, it's only available as complete units, bot parts.
(I might be mistaken on that one, but that is my understanding)
History fails to record a single precedent in which nations subject to moral decay have not passed into political and economic decline. There has been either a spiritual awakening to overcome the moral lapse, or a progressive deterioration leading to ultimate national disaster.
If you think a strong military is expensive, think about what a weak one would cost